Piezoelectric property is a property such that crystals of a material are electrically polarized corresponding to an external stress applied to the material. With the piezoelectric property, mechanical energy can be transformed to electric energy, and vice versa. Generally, a piezoelectric constant d.sub.31 is used to evaluate the piezoelectric property of the material and a larger piezoelectric constant means better transformation efficiency. Therefore, a material having a larger piezoelectric constant is preferred as the piezoelectric material.
Typical piezoelectric materials are roughly classified into ceramics (e.g. PbTiO.sub.3. BaTiO.sub.3, etc.) and polymers. Although ceramics have a large piezoelectric constant, they have some drawbacks in that they are less flexible, heavy, easily cracked and difficultly made into a thin film. Due to these drawbacks, their applications are restricted despite of their large piezoelectric constant.
On the contrary, polymeric piezoelectric material has a smaller piezoelectric constant than ceramics. However, polymeric material can be made into a thin film having a large surface area and mass produced.
Among the polymers, generally fluorine-containing crystalline polymers, for example, vinylidene fluoride containing polymers have a comparatively large piezoelectric constant. Recently, fluorine-containing polymers are, therefore, used in an acoustic field as a component of a head phone or a loudspeaker for high frequency. Further, composite materials having both characteristics of ceramics and the polymer are developed and used as, for example, a sensor.
The piezoelectric material, whether it is ceramic or a polymer, must be polarized to make it piezoelectric.
A thermal electret method is the most common method for polarization of the material it and comprises vacuum metallizing both surfaces of the material with metal such as aluminum to form electrodes, heating it at a high temperature, for example at 130.degree. C., applying a high electric field, for example 100 MV/m to it for a predetermined time, for example from 30 minutes to one hour, and cooling it to a room temperature under the high electric field. Since the thermal electret method requires a high electric field, long time and a high temperature, its productivity is low and it consumes a large amount of energy.
It has now been found that the vinylidene fluoride containing polymer is easily made piezoelectric at a low temperature in a short time by applying an alternating electric field (except a rectangular wave) corresponding to an electric field strength of 50 to 200 MV/m to the polymer.